Chemical plating process



United States Patent 3,346,404 CHEMICAL PLATlNG PROCESS George S. Gardner, Elkins Park, and Albert J. Saukaitis,

Wayne, P2,, assignors to Amchem Products, Inc., Ambler, Pa., a corporation of Delaware No Drawing. Filed Jan. 26, 1965, Ser. No. 428,241 Claims. (Cl. 106-1) ABSTRACT OF THE DISCLOSURE There is described a liquor finishing process for applying a copper-tin alloy coating to ferriferous metal surfaces by utilizing an aqueous acid sulfate solution containing stannous ions, copper ions, and particular amino acids, including glutamic, aspartic and glycine acids. There is also described replenishing of the liquor finishing solution.

This invention relates to What is known in the art as chemical plating, i.e., to a plating process which does not employ electric current. This type of process, when used to apply a binary alloy of copper and tin to the surface of a ferriferous metal substrate, is known in the industry as liquor finishing and this term will be employed in the following description and claims.

A liquor finish has been described as a coppered or brass colored finish such as obtained from the coppertin coating mixture which is used to treat iron or steel articles. Descriptions of this process may be found on pages 685 and 713 of The Making, Shaping and Treating of Steel, 7th edition by United States Steel Corporation.

Liquor finishing processes of the kind here involved have employed acid sulfate baths or solutions and have long been used on iron and steel surfaces, and particularly on wire surfaces, in order to facilitate subsequent cold drawing operations as well as, in some instances, to enhance the decorative appearance of the surface.

Wire which has been coated with a copper-tin binary alloy is particularly suitable for use in manufacturing tires since the synthetic tire components adhere more tenaciously to the copper-tin or so-called liquor finish alloy than they do to bare or untreated wire surfaces or to wire surfaces which may have been treated by other chemical coating processes.

Since its inception the art referred to has encountered several serious drawbacks. These include what are sometimes prohibitively high costs involved in producing the desired copper-tin alloy coatings as well as an inability to obtain consistently uniform coatings, especially when it is attempted to employ the coating baths over relatively long periods of time. In this connection experience has shown that, while freshly prepared liquor finishing baths produce uniformly acceptable binary alloy coatings of copper and tin, after relatively short periods of use the baths become totally incapable of maintaining the desired coating uniformity and quality despite all the attempts which have been made to revitalize them. Indeed, the only technique which seems to have been measurably sucessful has involved early discard of the bath and frequent replacement thereof with freshly prepared solution. This has been the general rule and such frequent discard and replacement of undepleted and slightly used baths has proven to be very costly due to the loss of substantial quantities of the essential coating constituents, particularly tin, which are dissolved in the baths. Liquor finishing, therefore, has been an expensive technique and the art has long needed a more economical substitute for present practices.

With the foregoing in mind the principal object of the present invention may be said to reside in the proice vision of a liquor finishing process of the general character described in which the plating bath employed can be utilized and maintained for long periods of time under conditions of heavy bath loading without frequent discard and replacement while at the same time making it possible to apply completely satisfactory and uniform binary alloy coatings of copper and tin on ferriferous metal surfaces with a consequent substantial reduction in the amount of chemicals consumed as well as in the cost of operation.

How this objective is attained, together with such other objects as may become apparent from the following disclosure will now be described in detail.

The present invention is based upon the discovery that an acceptable, highly uniform copper-tin binary alloy coating can be obtained on a long sucession of steel surfaces if such surfaces are subjected to the action of an aqueous acid sulfate solution consisting essentially of:

(a) from 1 to 15 grams-liter of dissolved stannous ion, calculated as Sn;

(b) dissolved copper ions, the weight ratio of copper to stannous ion being maintained between 0.1 and 0.8 as determined by the formula Cu+Sn and (c) from 0.05 to 5 grams/ liter of an amino acid selected from the class consisting essentially of glutamic,

'aspartic and glycine, where such solution is maintained within the ingredient proportions specified and at a pH of between 0.1 and 2.0 and, further, where the coating is applied with the bath at a temperature lying between from 15 C. to 66 C. The treatment, of course, is to be continued until the desired coating is formed but the length of the treatment may vary greatly depending in large measure upon such factors as bath temperature, solution concentration and acidity within the ranges specified. Generally, for a given length of treatment, where a low solution temperature is employed, or where low chemical concentration is used, the coatings obtained will be light in weight so that longer treatment periods may be desirable. Conversely, Where higher temperatures and/ or solution concentrations are employed, the coating weights will be relatively heavier per unit of treatment time. Accordingly, the length of treatment is primarily dependent upon the processing factors of temperature and concentration. Suflice it to say, however, that treating periods of from 1 to 10 minutes have been found to be completely satisfactory when operating within the teachings of this invention.

As noted above the stannous ion, calculated as Sn, must be present in amounts of from 1 to 15 grams/liter of the coating solution. Where the stannous ion concentration falls below or rises above this range, the coatings obtained will not be uniformly acceptable, will lack the essential adhesion properties when utilized in tire manufacture, and will not display the customary brassy appearance which is desired for this type of coating.

Dissolved copper ion must also be present in the chemi cal plating solutions of this invention and the amount of such copper ions which is to be employed will depend upon the level of stannous ion employed in accordance with the formula times in the operating solution, of a weight ratio of copper to stannous ion between 0.1 and 0.8.

3 As the weight ratio of copper ions to stannous ions, as determined by the formula is decreased below the minimum level of 0.1 thus reflecting a decreased concentration of copper in the coating solution, the coatings will be found to be very thin and of little commercial value, particularly so far as concerns utilization of the coatings in the tire manufacturing industry. Conversely, if the amount of copper ions dissolved in the solution is such as to cause the ratio of copper to stannous ions, as calculated from the formula to exceed the upper ratio of 0.8, then the coatings obtained will be appreciably heavier in weight and will be unacceptable from an appearance standpoint in that they will lack the customary brassy appearance.

Inasmuch as the chemical plating reaction must be effected from an acidic sulfate solution containing the dissolved ions noted above, the stannous and copper ions are preferably supplied by utilization of their respective metal sulfate salts. Moreover, the utilization of stannous and copper sulfate salts provides a readily soluble form of the essential ionic constituents and these salts represent readily commercially available sources of these constituents.

The acidity of the copper-stannous ion containing chemical plating bath is critical and, as noted hereinabove, must be maintained within the pH range of from 0.1 to 2.0. If the bath acidity is allowed to fall below the minimum pH value of 0.1, the bath will produce extremely thin coatings, if indeed any coatings are produced at all. Conversely, if the chemical plating solution acidity exceeds the pH value of 2.0 the coatings produced will be substantially non-adherent with respect to the base metal surface.

During the course of operating the process of this invention it may be necessary from time to time to effect pH adjustments -by the addition of acidic components, such, for example, as sodium acid sulfate or sulfuric acid for the purpose of maintaining the pH within the essential range. However, such acidic components must not include halide ions, particularly chloride ions, because it has been found that the presence of such ions in the chemical plating solutions of this invention will have an adverse effect upon the coating formation, and will result in totally unsatisfactory coatings having a predominantly copper content, if indeed any coating at all is produced on the metal surfaces being treated. If necessary, aqueous sodium hydroxide solutions may be used to raise solution pH values. pH measurements are determined by standard glass electrode measurements in accordance with normal chemical procedures.

As noted hereinabove. it has been found essential for satisfactory operation of the process of the present invention that from 0.05 to 5 grams/liter of an amino acid selected from the class consisting essentially of glutamic, aspartic and glycine acids be dissolved in the coating solutions. The presence of these acids has been found to be essential for the production of coatings having both uniform appearance and desired adhesion properties especially for use in tire manufacture. Where the amount of the amino acid falls below 0.05 gram/liter, subsequent coatings will be found to be of very light weight and of very poor appearance in that they will be uneven, and will be unacceptable for subsequent drawing or deforming operations. Conversely, if the amount of amino acids is permitted to rise above 5 grams/liter it will be found that such acids will have an adverse effect upon coating formation, and will cause precipitation of essential stannous ions from the coating bath.

While one or more of the required amino acids may be incorporated into the chemical plating solutions of this invention through utilization of essentially pure amino acids, it has been found that a desirable and economical source of such acids is commercial gelatin. Apparently as a result of acidic reaction on fibrous proteins, the gelatin molecule is hydrolyzed to yield one or more of the essential amino acid components. However, not all of the amino acids which may be derived from the hydrolysis of fibrous proteins have been found to be suitable for use in this invention. For example, use of either leucine or argenine failed to provide the uniform, high quality coatings obtained with the process of this invention. Where gelatin is employed the amount which should be utilized may range from 0.2 to 20 grams/liter and this will insure the provision of the required amount of amino acid constituent. Nevertheless, despite the fact that hydrolysis of fibrous proteins may yield leucine and argenine the presence of these two amino acids does not interfere with the operation of the process in the manner desired because such hydrolysis also produces the desired amino acids as specified above, namely, those which are chosen from the class which consists of glutamic, aspartic and glycine acids. In other words, sufficient of the necessary amino acids will be present in the bath despite the possibility that leucine and argenine may also be present. However, leucine and argenine by themselves will not satisfy the requirements of this invention.

With respect to coating temperature, it is essential that the liquor finishing bath 'be operated between 15 C. and 66 C. If the bath falls below 15 C. the coatings subsequently obtained will be very light in weight and totally unsuitable for use either in subsequent drawing or deforming operations or in tire manufacture. Conversely, if the bath temperature is permitted to rise above 66 C., the coatings will become very heavy and the rate of loss of stannous ions through oxidation will increase with corresponding increases in the amount of sludge in the coating bath and a concomitant waste of essential stannous ions.

A word of caution should be introduced on the matter of possible'agitation of the plating bath. Experience has indicated that beyond what might be called a normal degree of agitation, such as is associated with the treatment of the work, excessive unnecessary agitation should "be avoided wherever possible because agitation tends to interfere with the production of coatings which have the desired brassy appearance. Furthermore, when the bath is subjected to increased turbulence due to an undue degree of agitation the resulting coatings tend to have very poor adhesion properties and frequently consist predominantly of copper.

In order to illustrate the improved coating process of this invention, and its ability to produce highly uniform copper-tin binary all-0y coatings at low overall operating costs on a long succession of pieces, there is presented below the details of a preferred operating procedure wherein both cold and hot rolled steel surfaces were subjected .to the action of a solution falling under the teachings of this invention.

In the first place, the following Example I illustrates the bath which was employed:

EXAMPLE -I 450 gallons (1703 liters) of an aqueous solution was prepared so as to contain:

Grams Lbs. G./1.

19, 686 43. 4 ll. 56 14, 198 31. 3 8. 3 25, 402 56. O 14. 9

The solution just referred to had a pH of 1.3 and was heated to 60 C. Both cold and hot rolled steel wire, which previously had been pickled in sulfuric acid, were then subjected to the action of the above liquor finishing bath and the treatment was continued for periods of 5 minutes, following which the treated Wires were subjected to a waternnse. This procedure was continued over a long succession of wire surfaces and periodic replenishment of the bath was accomplished by utilizing suitable quantities of the following dry formulation:

FORMULATION I Percent by weight SnSO 25 CuSO J-I O 5 0 Natl-I50 24.5 Gelatin Powder 0.5

Ratio =0.56

Cu Cu Sn Certain pertinent operating information which was accumulated during this test is summarized in the following table:

TABLE I Total Wt. of Wire Treated, lbs.

Coating Bath Analysis, g./l.

Fe Cu Sn The heavy coatings produced were completely acceptable for drawing or cold deforming operations.

The analytical procedures utilized in determining the iron, tin and copper content of the bath were as follows:

Iron

Titration of a bath sample to which has been added sulfuric acid and mercuric chloride, with 0.1 N potassium permanganate solution to a permanent endpoint. The amount (mls) of titrating solution required is Titration A.

Tin

The titration of a bath sample with 0.1 N potassium permanganate solution to a permanent endpoint. The amount (mls) of titrating solution required is Titration B. The desired tin titration value is then obtained by subtracting the Titration A value from this Titration B value.

Copper Titration of a bath sample, after hydrogen peroxide oxidation of the stannous and ferrous ions, with 0.1 N sodium thiosu-lfate solution to a permanent endpoint.

At this point we wish to say that replenishment of the plating bath during the time that it was in use could have been made by individual additions of each ingredient so long as this was done in a way to maintain their proportionate relationship as given near the beginning of this specification. The bath of Example I falls within the relationship given as will be quickly apparent from examination of the last column of the example, the gelatin powder called for yielding from 0.05 to 5 grams/ liter of amino acid from the desired class of glutamic, aspartic and glycine acids. However, we prefer to employ a formulation such as given under the heading Formulation I because the quantities of ingredients called for in this formula reflect the changes in the proportionate relationship of the several ingredients as it is necessary to add them in order to maintain the original make-up relationship even after the bath has been in use for relatively long periods of time and has been employed in the coating of a long series of surfaces. Of course, the quantities of Formulation I which :are added to maintain the bath of Example I must bear proper relation to the quantities of chemicals consumed and this is set forth in Table I.

The technique described has resulted in outstanding improvement in the liquor finishing art. It has yielded completely acceptable, tightly adherent, and unusually uniform coatings over a long period of time and in the coating of large areas of metal.

Additional illustrations of the operation of the process of the present invention are contained in the following table wherein each example reported utilized steel wire which was pickled in sulfuric acid prior to treatment in the solutions of this invention.

TABLE II Amino Acid Ratio Cu/ Cu+Sn Sn, g./1. D

Treatment, C.

Wt.,mg./it. Appearance Adherence QNJNNUIOOLONlQUILOMl d0 Glutamic do Aspartim--- Glycine..-

Cold rolled. Do. Hot rolled. Do. Cold rolled. D0.

0. Hot rolled.

D0. D0. Cold rolled. Do. Do. Do. Hot rolled. D0. Cold rolled. Hot rolled.

0. Cold rolled. Hot rolled.

Do. Hot rolled.

Do. Cold rolled. Do. Do.

While Formulation I has been found to be an exceedingly satisfactory dry admixture for use in effecting replenishment of the plating bath we have found that some reasonable variation in the proportionate relationship of the dry ingredients can be adopted in preparing a replenishment powder. For instance, it is quite practicable to employ dry admixtures which contain from 1% to amino acid from the class above described or of commercial gelatin; from 9% to 85% of sodium acid sulfate; and from 90% to of an admixture of SnSO and CuSO Of course, the amounts of stannous and copper sulfates employed in the dry formulations must be such as will yield the desired ratio of Cu Cu +Sn and this must lie between 0.1 and 0.8 when thedry replenishment material is dissolved in the coating bath.

In order to illustrate various examples of dry replenishment formulations falling within the limits just set forth there are presented below several additional formulation admixtures which are presented strictly by way of exemplification rather than by way of limitation.

FORMULATION II Percent by weight Glutamic acid 1.0 NaHSO 9.0 CuSO .H O 37.8 SnSO 52.2

Cu B31310 m-QQZ Utilization of 7 grams of Formulation II per liter of coating solution results in the addition of approximately 0.07 gram/liter of glutamic acid; 0.63 gram/liter of NaI-ISO 2.64 grams/liter of CuSO .I-I O; and 3.66 grams/liter of SnSO If 52.5 grams of Formulation II are utilized per liter of coating solution, there will result approximately 0.52 gram/liter of glutamic acid; 4.73 grams/ liter of NaI-ISO 19.8 grams/liter of CuSO .H O; and 27.45 grams/liter of SnSO FORMULATION III Percent by weight Gelatin 5.0 NaHSO 85.0 CuSO .H,-;O 4.2 snso. 5.8

Cu Raf/10 -Cu+S 0.32

Where 62.5 grams/ liter of Formulation III are utilized in replenishing coating solutions, such action will supply 3.1 grams/liter of gelatin; 53.1 grams/liter of NaHSO 2.6 grams/liter of CuSO .H O; and 3.7 grams/liter of SKIS-O4.

FORMULATION IV Percent by weight SnSO 27.0

RatiO =O.55

Cu Cu+Sn We have found that our invention offers a readily controllable process which produced uniform copper-tin coatings having good drawability and especially excellent qualities with respect to adhesion of the coatings to rubber. It has also been discovered that the long bath life characteristic of our invention results in less down time and appreciably greater longevity for the chemical plating solutions which are prepared and maintained in accordance with the teachings of this disclosure.

We claim:

1. The method of liquor finishing a succession of ferriferous metal surfaces which comprises subjecting the surface to the action of an aqueous acid sulfate treating solution consisting essentially of:

(a) from 1 to 15 grams/liter of dissolved stannous ion, calculated as Sn;

(b) dissolved copper ions in a weight ratio to the stannous ion of from 0.1 to 0.8 as determined by the formula Cu-l-Sn and (c) from 0.05 to 5 grams/liter of amino acid chosen from the class which consists of glutamic, aspartic and glycine acids;

maintaining the solution at a pH of from 0.1 to 2.0, and at a temperature of from 15 C. to 66 C.; continuing the treatment until a coating is formed; and periodically analyzing and replenishing the solution as required to maintain its essential ingredients within the ratios specified.

2. The method of claim 1 wherein the amino acid content is supplied by an appropriate quantity of commercial gelatin.

3. The method of claim 2 wherein the quantity of gelatin employed is from 0.2 to 20 grams/liter.

4. The method of claim 1 wherein the stannous and copper ions are supplied by using their sulfate salts as the source thereof.

5. The method of claim 1 wherein the length of the treatment is from 1 to 10 minutes.

6. The method of claim 1 wherein replenishment of the treating solution is effected by appropriate periodic additions of the following dry formulation:

Percent by weight SnSO 25 CuSO .H O 50 NaHSO 24.5 Gelatin powder 0.5

7. The method of claim 1 wherein replenishment of the treating solution is effected by appropriate periodic additions of the following dry formulation:

Percent by weight NaHSO from 9 to Gelatin powder from 1 to 5 Admixture of SnSO and CuSOA from to 10 of from 0.1 to 0.8.

8. A liquor finishing aqueous acid sulfate solution consisting essentially of (a) from 1 to 15 grams/liter of dissolved stannous ion, calculated as Sn;

(b) dissolved copper ions in a Weight ratio to the stannous ion of from 0.1 to 0.8 as determined by the formula Cu Cu+Sn Percent by Weight SnSO 25 CuSO .H O 50 NaHSO 24.5 Gelatin powder 0.5

10. A dry admixture for replenishing aqueous acid sulfate liquor finishing baths consisting ese-sntially of the following ingredients in the proportions indicated:

Percent by weight NaI-ISO from 9 to Gelatin powder from 1 to 5 Admixture of 51150 and CuSO from to 10 with the amounts of stannous and copper sulfates being such as to yield in the bath a ratio of Cu-l-Sn of from 0.1 to 0.8.

References Cited UNITED STATES PATENTS 510,376 12/1893 Bertrand 1061 2,472,393 6/1949 Avallone et a1 l06-1 2,519,672 8/1950 Lawless 1061 3,050,410 8/1962 Greene 1061 3,246,995 4/1966 Moore 1061 ALEXANDER H. BRODMERKEL, Primary Examiner.

L. B. HAYES, Assistant Examiner. 

8. A "LIQUOR FINISHING" AQUEOUS ACID SULFATE SOLUTION CONSISTING ESSENTIALLY OF: 